Make theme values passed to css and sx properties typesafe

[ammar88]

Is your feature request related to a problem? Please describe.
I'd like to find a way to make the value I pass to variant (and other theme aware properties) type safe and have it provide autocompleted values from my theme. Currently I can pass any string to variant and it will fail silently since variant accepts any string.

<SomeComponent sx={{ variant: 'accepts any string and produces no error' }} />

Describe the solution you'd like
I'd like to able to pass a string that corresponds to what exists in my theme file and have errors produced if I pass a string that doesn't.

Additional Context
Would this solve my issue?

[lachlanjc]

Thanks for the issue. We are working on this in #1090, but it’s been a big project. Help appreciated!!